Mixed Air Calculator

A mixed air calculator describes what happens when two flowing air streams meet and reach a common condition. This is a routine problem in heating, ventilation, and air-conditioning design. An air-handling unit may blend cool outdoor air with warm return air before a heating or cooling coil. Meteorologists also combine air parcels to estimate the temperature and moisture state of a mixed layer. Because moist air contains both dry air and water vapor, simply averaging temperature and relative humidity is not physically correct. This calculator first converts each temperature and relative humidity pair into a humidity ratio. Saturation vapor pressure is estimated with a standard exponential correlation, actual vapor pressure is found from relative humidity, and humidity ratio is calculated as kilograms of water vapor per kilogram of dry air. The tool then calculates moist-air enthalpy for each stream. Both humidity ratio and enthalpy are averaged using dry-air mass flow, so a stream carrying twice as much dry air has twice the influence on the final condition. The mixed dry-bulb temperature is recovered from the combined humidity ratio and enthalpy. Mixed relative humidity is then calculated from the resulting vapor pressure and the saturation pressure at that temperature. This energy-and-moisture balance is more accurate than separately averaging temperatures and humidity percentages. The displayed humidity ratio is converted to grams of water per kilogram of dry air, while enthalpy is shown in kilojoules per kilogram of dry air. All streams are assumed to mix adiabatically at the same atmospheric pressure, with no condensation, evaporation from surfaces, fan heat, or external heat transfer. If the theoretical result exceeds saturation, real equipment can form condensate and the final state requires a saturated-air and liquid-water energy balance. Inputs should also represent dry-air mass flow rather than total moist-air flow when high precision matters. At ordinary indoor conditions the numerical difference is small, but engineering specifications should use a consistent basis. Use standard atmospheric pressure of 101.325 kPa near sea level, or enter a measured local station pressure for higher elevations and weather analysis. The result is useful for outdoor-air ventilation studies, economizer control, make-up air systems, laboratory exhaust replacement, drying processes, and classroom psychrometric exercises. It gives a transparent first estimate that can be checked against a psychrometric chart or detailed HVAC simulation.